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An 11-year-old from Michigan said he is going to really wow his schoolmates Friday with the "coolest" show-and-tell item anyone's ever brought to the sixth grade.

After all, it's not every day you get to show off a 13,000-year-old mastodon bone you and your cousin found in a stream behind your backyard.

"I thought it was a rock at first, but a couple minutes later I looked more at it, and I didn't think it was a dinosaur bone, but I wasn't sure," Eric Stamatin of Shelby Township Michigan told CNN on Thursday.

He and Andrew Gainariu, 11, from Troy, Michigan, were hunting for crayfish in the stream that extends from the middle branch of the Clinton River, as they often did, when they "got bored" and decided to build a dam.

They made an extraordinary discovery that June day.

John Zawiskie, a geologist and paleontologist at the Cranbrook Institute of Science confirmed in early November that what the "kids just being kids outside" discovered was not a rock at all, but an axis, a specialized second vertebrae behind the skull in the spinal column of an American mastodon.

"These animals have been extinct for 10,000 years," Zawiskie told CNN. He works extensively on subjects in this time period, and noticed the distinct bone from the prehistoric elephant-like mammal right away.

"Oh my gosh, we were like psyched," Stamatin said of learning it was a real mastodon bone. "I was like I think we are the first people in Michigan to find a bone (who) are only 11 years old."

The mastodon, which is Michigan's state fossil, is common to the geographic landscape, according to Zawiskie. He said more than 200 discoveries have been made in the southern half of Michigan over the past 150 years, usually by farmers or construction crews.

The large mammals became extinct because of nomadic hunters causing new stress for the population and a very abrupt climate change, dubbed the "Younger Dryas" period, when temperatures reversed to glacial conditions.

"There are just incredible stories beneath us, about how Earth changed in the past," Zawiskie said.

"Every once in a while -- by construction workers, or exploring 11-year-old boys and girls -- something like this comes to the forefront and reminds us of the incredible history of Earth's landscape."

Cristina Stamatin, Eric's mother, said when the boys stumbled in the house with the discovery her first thought was "oh my goodness, what did they bring home?"

She told CNN she thought it was a cow's bone. "It's been sitting on dining room shelf since last June," Stamatin said. She reached out to the paleontologist in November.

"John, Dr. John the scientist as my son calls him, was over my house within an hour after me e-mailing him to say 'you won't believe what you discovered'" the mother explained.

When they first discovered the mass with a large hole in it, Andrew Gainariu told CNN, "I said it's obviously a dinosaur bone, but my cousin said 'no.'"

"I told Eric, you really want to go there? I bet 100 bucks it is a dinosaur bone, because I have always been fascinated with bones," Gainariu continued.

The bet hasn't been settled because the boys have since learned it's not a dinosaur bone, but that of an extinct mammoth.

Gainariu said this experience has him thinking about maybe becoming a paleontologist, but he is still considering being a lawyer "and other things too."

Stamatin said he is a bit nervous about his presentation Friday.

"I am bringing the bone tomorrow, and I am going to make a poster and explain to every science class in sixth grade what a mastodon is," he told CNN. "I feel really special. People said it's just amazing how I found this."

The boys are mulling over what they are going to do with the fossil.

"Andrew said maybe we can sell it, and that we could be famous... but I guess we are famous already," Stamatin said.

"We are still in the thinking process of putting it on eBay or giving it to the museum," Gainariu clarified Thursday.

"I have been thinking of selling it and making money off it," Gainariu continued, "but who is going to pay 2,000 bucks for a bone?"